In general, materials are isotropic or anisotropic. Isotropic materials have identical properties in all directions. Conversely, properties of anisotropic materials are directionally and geometrically dependent.
Conventionally, in applications that require the use of materials that permit bending, materials such as metal, polymers, or composite can be used to form flexural beams. The modulus of the materials used in the beams and their geometry influence the stiffness. Depending on the material used to construct the beam, the beam can have isotropic or anisotropic properties. Nevertheless, beams constructed with such materials have an inherent inability to exhibit a low bending resistance in one direction and a high bending resistance in the other. Furthermore, these materials exhibit a linear relationship between stress and strain.
Many products, such as consumer products, including footwear and apparel, medical devices, medical appliances, manufacturing products, and many other products, incorporate materials to provide a selected degree of stiffness while still allowing for some flexibility for bending during use. However, oftentimes, desired characteristics within a shoe or other similar products can be at odds with other desired characteristics. For example, footwear often incorporates materials that allow the sole assembly to bend and flex with a wearer's foot during use, while also providing a desired level of protection and structural stability to the foot. For example, a sole assembly construction that provides enhanced flexibility is often provided at the sacrifice of structural stiffness and or stability. Conversely, the use of materials to provide enhanced structural stiffness and stability are often at the sacrifice of flexibility.
In materials with a linear relationship between stress and strain, stiffness is constant. However, in many applications, materials that increase stiffness as a function of strain are desirable. For example, in some products such as footwear, it is desirable to allow the footwear to bend in the toe region to allow the wearer's toes to bend through a normal range of motion. It is also desirable, however, for the footwear to provide a stiffness that prevents the toe region from bending past the normal range of motion resulting in a condition of increased strain, thereby avoiding hyperextension of the wearer's toes (i.e., turf toe). Similarly, it is desirable to provide a brace or other medical appliance that allows for bending or articulation of a portion of a wearer's body through a normal or selected range of motion. It is also desirable, however, to provide a stiffness that prevents articulation of the body portion beyond the normal or selected range of motion, which would create a condition of increased strain. The present technology can achieve this desirable configuration that increases stiffness as a function of strain.
Linear stiffness behavior and tradeoffs between competing performance and operating characteristics is often encountered in the manufacturer and/or use of a wide variety of products. Accordingly, there is a need for a material suitable for applications requiring variable modulus material (i.e. anisotropic flexural material or strain stiffening material).